1. Field of the Invention
The present invention relates to spectroscopy. In particular, the present invention is an improved method and apparatus for exciting ions into resonance within an ion cyclotron resonance mass spectrometer.
2. Description of the Prior Art
Ion Cyclotron Resonance Mass Spectrometry (ICR/MS) is a well known technique for detecting gaseous ions and is described in U.S. Pat. Nos. 3,742,212 to McIver, Jr. and 3,937,955 to Comisarow et al. Gaseous ions formed from a sample are trapped within an analyzer cell by a static electric field. These ions are subjected to a magnetic field and are thereby constrained to move in circular orbits in a plane perpendicular to the magnetic field. The frequency of the orbital motion is termed the "natural cyclotron frequency", and for any given ion is dependent upon the mass and charge of the ion, and the strength of the magnetic field.
Ions to be analyzed are then excited into coherent orbits through the application of a radio-frequency (rf) electric field. Ions whose natural cyclotron frequency is matched by the frequency of the applied rf electrical field will absorb energy from the electric field and be accelerated to larger orbital radii and higher kinetic energy levels. These ions are said to be in resonance.
As the ions resonate within the analyzer cell, an image current is induced in electrode plates positioned on opposite sides of the cell. The image current is detected and converted to a frequency-domain spectrum whose peaks can be correlated with the mass-to-charge ratio and abundance of the gaseous ions being analyzed. Ions of different mass to charge ratios have different resonant frequencies they can be distinguished one from another.
In a typical ICR/MS instrument, the ions are excited into resonance by a swept-frequency rf electric field. The electric field is produced by a swept-frequency rf signal which is applied to electrode plates positioned on opposite sides of the analyzer cell. A swept-frequency signal is one having a frequency which increases or otherwise varies with time. The frequency of the rf signal is usually made to increase linearly with time, although other functions, such as a logorithmic variation, can be used. This prior art excitation function is described by the following formula: EQU E.sub.Excitation =E.sub.o sin((F.sub.o +F'.sub.t)t+.theta..sub.o)
where:
E.sub.Excitation is the excitation signal applied to the ions. PA1 E.sub.o is the amplitude of the excitation signal. PA1 F.sub.o is the initial frequency of the excitation signal. PA1 F' is the rate of change in frequency of the excitation signal. PA1 t is time. PA1 .theta..sub.o is the initial phase of the excitation signal.
The amplitude, E.sub.o, of the excitation signal is constant with time. The excitation function therefore has an envelope of rectangular shape which is applied for a period t.sub.0 to t.sub.l such that: ##EQU1##
A major problem encountered in mass spectral analysis of gaseous ions is the variation in the energy of the effective rf field used to bring the ions into resonance. A Fourier Transform of the excitation function described above reveals that the power spectrum varies significantly over the frequency range of interest. This is due to the rectangular envelope of the swept rf signal and field which abruptly switch on and off at times t.sub.0 and t.sub.1, respectively. These variations in the power spectrum produce a lack of uniformity in the energy imparted to ions of differing frequencies. Errors in the determination of physical parameters of the ions, such as mass and abundance of ions of a given mass-to-charge ratio, are a direct consequence. This in turn affects the accuracy of ICR/MS measurements. To compound matters, it is known that ions of different orbital radii are affected differently by the rf electric field. Experiments have shown that perturbations in the natural cyclotron frequency of these ions is due in part to variations in the power spectrum of the excitation signal.
The problems described above are well known and have been previously addressed. In addition to the excitation method described above, the Comisarow et al Patent suggests several alternatives. One such method involves applying a sine-wave pulse to one of the electrode plates of the analyzer cell. It is stated that the Fourier Transform of this pulse is a frequency function which is essentially flat over the frequency range .+-.1/4.tau. Hz centered at the frequency of the sine-wave pulse. A second and related alternative is to apply a dc pulse having a duration of about 100 nsec. It is suggested that it is possible to achieve an essentially uniform irradiation field over a frequency range from about dc to about 2M Hz by the application of such a pulse.
Neither of these alternatives is workable. As the specification of the Comisarow et al Patent notes, the amplitude of such pulses must be very large if they are to be adequate to excite ions over the entire frequency range. As a practical matter it is virtually impossible to use this technique for this very reason. Furthermore, these methods would produce no better power uniformity than the swept-frequency approach.
The determination of relative abundance of ions is based on the strength of the image current observed for each given ionic species present in the analyzer cell. Accuracy of this determination requires that each ion be subjected to excitation of the same effective intensity from the rf electrical field. Clearly, currently known apparatus and methods for exciting ions have power spectra which vary over frequency. It would be desirable to excite the ions with an excitation signal which is constant with frequency. The result would be a significant increase in the accuracy of ICR/MS measurements.